Film clamp for film sealing apparatus and method

ABSTRACT

Film clamp for the lower or stationary jaw of a sealing machine, particularly the lower or stationary jaw of an L-bar sealing machine. Preferably the film clamp is constructed of a thermally conductive material and is configured to effectively dissipate heat. A closed loop feedback system can be used to regulate the applied power to the sealing element in order to maintain its temperature within a predetermined range to help prevent overheating of the film clamp and other jaw components.

BACKGROUND OF THE INVENTION

[0001] Packaging machines for wrapping and sealing plastic film about anarticle conventionally utilize a heated wire to seal film layers to oneanother and to melt through the layers in order to separate one articlefrom another as the articles pass through the machine. Current issupplied to the wire to heat the wire to a high temperature in order toeffect the seal and cutting operation. The ideal appearance of theresulting seal is fine and neat as the film shrinks tightly around thepackage, especially where polypropylene films are involved. Such hotwires are typically used to form both end seals and side seals.

[0002] L-bar sealers are generally used to seal heat shrinkable plasticcenterfolded film around an object to be packaged. The object to bepackaged is wrapped with the film, and the L-sealer top jaw is loweredto cut and seal together abutting surfaces of the centerfold film. TheL-bar makes two seals at one time, sealing both the edge of thecenterfold wrap and also cutting and sealing across the width. Filmssuch as polypropylene, PVC and polyethylene are typically sealed withL-bar sealers. The sealers can be manual, semi-automatic or fullyautomatic.

[0003] As the sealing element contacts the film and performs itsintended function, it loses heat to the film as well as to thesurrounding environment and near-by components. Accordingly, currentmust be continually or continuously supplied to the sealing element inorder to maintain the appropriate temperature to perform the intendedoperation. However, heating the sealing element causes a correspondingtemperature increase in adjacent and near-by components as well as theframework of the apparatus, which can be highly undesirable. Indeed,where the framework is constructed of a polymeric material, excess heatmigrating into the polymeric components can melt the same.

[0004] This heating of adjacent and near-by components has in some casesresulted in the elimination of the use of film clamps in the uppersealing jaw. Although such film clamps are desirable to hold the film inplace during sealing, if such clamps become too hot, film in contactwith the clamps will be heated and can begin to shrink prematurely.Premature film shrinking can result in poor package appearance or even acomplete failure of the packaging process.

[0005] It therefore would be desirable to provide a seal system for anL-type sealing apparatus that utilizes film clamps and prevents theiroverheating by maintaining the temperature of the seal element within adesired range.

[0006] It further would be desirable to provide a film clamp in thelower or stationary jaw of an L-bar type sealing machine to effectuate aconsistently high quality seal.

[0007] It further would be desirable to provide a film clamp that keepsthe film from draping over the sealing element upon contact, and fromdepositing residue and potentially overheating and burning seals.

[0008] These and other objects will be made apparent by reference to thefollowing description and drawings.

SUMMARY OF THE INVENTION

[0009] The problems of the prior art have been overcome by the presentinvention, which provides a film clamp for the lower or stationary jawof a sealing machine, particularly the heated lower or stationary jaw ofan L-bar sealing machine. Preferably the film clamp is constructed of athermally conductive material and is configured to effectively dissipateheat. A closed loop feedback system can be used to regulate the appliedpower to the sealing element in order to maintain its temperature withina predetermined range to help prevent overheating of the film clamp andother jaw components. The use of the film clamp resluts in the filmcontrol required for consistent seal quality, especially when sealingheavier gauge films such as CORTUFF® (3 mils thick) commerciallyavailable Sealed Air Corporation.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIG. 1 is a perspective view of a seal jaw in an L-seal assemblyin accordance with the present invention;

[0011]FIGS. 2 and 2A are views of a termination block of the seal jaw ofFIG. 1;

[0012]FIG. 3 is an exploded perspective view of the jaw assembly of theseal jaw of FIG. 1;

[0013]FIG. 3A is a cross-sectional view of the jaw assembly of FIG. 3;and

[0014]FIG. 4 is a view, partially in section, showing a seal being madein accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Turning now to FIG. 1, there is a shown a portion of an L-sealapparatus for a packaging machine in accordance with an embodiment ofthe present invention. The bottom jaw of the L-bar includes a front jawassembly 12 and a cross jaw assembly 10. For purposes of illustration,the cross jaw assembly 10 will be discussed to illustrate theconstruction of the L-bar, although those skilled in the art willappreciate that analogous components typically are used to make up thefront jaw assembly 12. In the preferred embodiment shown, the bottom jawassembly is stationary and the top jaw assembly (not shown) is movablefrom an open non-sealing position to a closed, sealing position thatsandwiches the film between the top and bottom jaws. Those skilled inthe art will appreciate that the bottom jaw assembly could be movableand the top jaw assembly stationary, or both the top and bottom jawscould be movable.

[0016] A termination block 14 (best seen in FIG. 2) is located at eachof the ends of the L-bar and holds the sealing element in place. To thatend, a sealing element is supported in slotted sealing element terminal15, and the terminal 15 rides in the block 14 to accommodate expansionand contraction of the sealing element due to its temperaturevariations. Compression spring 16 biases the terminal 15 inwardly asbest seen in FIG. 2A, thereby allowing the terminal 15 to accommodatethe expansion and contraction of the seal element. A TEFLON shieldedwire 13 is in electrical communication with the seal element and with acurrent supply source (not shown) for providing current to the sealingelement to heat the element to the desired temperature.

[0017]FIGS. 3 and 3A illustrate the various components of the bottomcross jaw assembly 10. Wave springs 17 support film clamp 18 on crossguard 19 as shown, and biases the film clamp 18 towards the film duringsealing. The cross guard 19 is preferably constructed of a conductivematerial, preferably aluminum, and acts as a heat sink (with the aid ofa plurality of projections 31 extending from its surface) to assist indissipating heat from the sealing element. The film clamp is preferablyan elongated linear substantially U-shaped member that can be formed byany suitable means, preferably by extrusion. Encompassed within the term“substantially U-shaped” is the provision for the opposite sides of filmclamp 18 to extend upwardly at an inward angle, and surround bottom jawmember 20 that sits inside the opposite sides on spacers 21. By sotapering the film clamp towards the seal element, a large portion of thefilm clamp (e.g., the lower portion of the clamp) can be positioned asfar away as possible from the hot sealing element to keep the clamprelatively cool without sacrificing the positioning of the top free endsurfaces of the clamp as close to the sealing element as possible tominimize film waste (the closer the surface(s) of the film clamp thatcontact the film are to the sealing element, the less film is used inthe packaging operation). Thus, the film clamp 18 includes a base 118and opposite sides 119, 120 extending therefrom that terminate inrespective free ends 18A and 18B. The spacers 21 support the bottom jawmember 20 away from the cross guard 19 and isolate heat away from thecross guard 19. Preferably the opposite sides 119, 120 of the clamp 18include one or more upwardly sloping projections 11 that serve as a heatsink to dissipate heat from the sealing element. The opposite sidesterminate in respective free ends 18A, 18B that provide the surfaceagainst which the film is sandwiched during sealing. These surfaceseffectively clamp the film being sealed in place, keeping it taut duringthe sealing operation to effect a high quality seal. These surfaces arepositioned higher than the sealing element 30 when the assembly is notin a sealing mode, so that when the top jaw assembly is forceddownwardly against the bottom jaw assembly, the film first contacts thefilm clamp before contacting the sealing element. A distance of about{fraction (1/16)} of an inch above the sealing element has been found tobe suitable. The wave springs 17 on which the clamp 18 is supportedallow for flexibility as the top seal jaw is compressed over the film tomake the seal, forcing the clamp 18 downwardly so that the film contactsthe seal element 30. The number and spacing of the wave springs 17 isnot critical as long as they effectively function to bias the clamp asdiscussed in greater detail below. The bottom jaw member 20 supportsinsulator 22 that is formed with a central slot (FIG. 3A) to hold thesealing element 30 and insulate it against heat and voltage.

[0018] Turning back to FIG. 1, corner block 23 is coupled to cornermount 24 by any suitable means, such as with slotted screws 25 and hexscrews 26. The corner block 23 is made of a thermally conductivematerial, preferably brass. A heat sensing element 37 such as athermocouple is positioned to sense a temperature that correlates to thetemperature of the sealing element. Since sensing the temperature of thesealing element directly is often difficult due to the lack of surfacearea of the sealing element, preferably the heat sensing element 37 ispositioned to sense the temperature of the corner block 23. Preferablythe heat sensing device is in direct contact with the corner block 23,although positioning the sensing device in close proximity to the cornerblock 23 also is suitable. The sensing device 27 is also in electricalcommunication with a controller board (not shown). The sealing elementis positioned in curved slot 29 of the corner block where it makes theturn from the cross jaw to the front jaw. The sealing element is in goodthermal contact with the corner block 23. Since the corner block 23 andsensing device 27 are centrally located along the length of the sealingelement (the cross jaw and front jaw are generally about the samelength), the temperature sensed by the sensing device 27 is an excellentindication of the average temperature of components of the jaw assemblythat are in contact with or are near the sealing element.

[0019] As set forth in co-pending patent application entitled “Apparatusand Method for Controlling Temperature of a Film Sealing Element” filedconcurrently herewith, the disclosure of which is hereby incorporated byreference, heat is input to the seal element 30 in an impulse mode. Forexample, at the point of jaw closure where the hot seal element ispressing the film layers against a sealing pad surface, the closure ofthe top jaws activates a detection circuit that then triggers a specificduration of pulse of current through the seal element 30 to increase thetemperature of the element to physically produce a seal and cut. Theimpulse replaces the heat that the seal wire loses during the sealingprocess, such as to the two webs of film and to the opposite sealsurface, and the heat lost to the surrounding environment, particularlybetween seals. The suitable amount of current for the impulse mode andthe duration of the impulse depend in part on the gauge and type of filmto be sealed and the desired quality of the seal. For example, a voltageof 42 volts and a current of 48 amps are suitable for films typicallyused in L-bar sealers of this type. The current is kept constant; onlythe duration of the current is modified in response to the sensedtemperature. Preferably the impulse duration may be from about 0.1seconds to about 1.5 seconds, most preferably about 1 second. The heatsensing element 27 continuously or continually monitors the temperatureof the mounting block 23. When the sensed temperature exceeds apredetermined value or range of values, the control board reduces theduration of the impulse a predetermined amount. One method fordetermining an appropriate temperature above which is consideredexcessive, thereby triggering the reduction of impulse current duration,is to determine the impulse current duration time set by the operator.From the impulse duration time selected, based on experience theoperator can estimate the appropriate temperature of the sealing element(based, for example, on the nature of the film and the operating speed),and a suitable temperature above this estimated temperature is chosenabove which would be considered excessive. A similar operation can becarried out to determine the minimum temperature necessary to effectuatea seal.

[0020] As a result, the seal element holder, the film clamps, theframework of the apparatus, and other adjacent components are keptrelatively cool. Indeed, because of the excessive temperatures occurringin the prior art devices, a film clamp either could not be used, or hadto be cooled with a cooling fluid that required extensive engineeringand was problematic.

[0021] Suitable sealing elements 30 include wires, bars and blades.Preferably the sealing element 30 is an elongated blade having a uniformcross-section. The uniform cross-section allows the blade to be turnedover in the event the cutting edge becomes dull, thereby doubling itsuseful life.

[0022] Turning now to FIG. 4, the operation of the film clamp is shownwith reference to the front jaw assembly. The film 100 is shown encasinga package 90 and extending beyond the package 90 between the top jawassembly 80 and bottom jaw assembly, specifically, the film clamp 18.The top jaw 80 pivots downwardly towards the bottom jaw and pressesagainst the film clamps 18 and seal blade 30, compressing the film underpressure to cut and seal the film. As the film 100 to be sealed ispushed down into the sealing element, it first contacts the film clamp18. As the force continues, the clamp moves downwardly against the biasof the spring(s) 17, causing the film to contact the sealing element,and the sealing element seals and cuts the film. During this operation,the film clamp 18 holds the film 10 firmly against the top jaw and in aplane to enhance the seal quality. As the sealing and cut progressthrough the film 10, sealed edges of the overwrap and selvage 50 drawaway from the heating element and each other in a controlled fashion,resulting in an optimal seal configuration. As the pressure is released,the film clamp 18 lifts the cut and sealed film up and away from theheating element (due to the biasing of the spring(s) 17). This liftkeeps the film from dragging and depositing undesirable residue on thesealing element, which conventionally required frequent cleaning. Inaddition, lifting protects the quality of the sealed overwrap. Extrafilm selvage 50 is cut off in the sealing process.

[0023] Preferably the film clamps of the present invention arepositioned in the jaw assembly housing the sealing element, which mostpreferably is the lower and stationary jaw assembly.

What is claimed is:
 1. Apparatus for sealing a plastic film, comprising:a top jaw assembly and a bottom jaw assembly housing a sealing element,said top jaw assembly and bottom jaw assembly being movable with respectto each other; a source of power to heat said sealing element to atemperature effective for sealing said plastic film; and a film clamp insaid bottom jaw assembly having at least one surface for positioningsaid film with respect to said sealing element.
 2. The apparatus ofclaim 1, wherein said bottom jaw assembly is stationary and said to jawassembly is movable.
 3. The apparatus of claim 1, wherein said filmclamp comprises a plurality of projections for dissipating heat.
 4. Theapparatus of claim 1, further comprising a spring in said bottom jaw forbiasing said film clamp towards said top jaw assembly.
 5. The apparatusof claim 4, wherein said biasing of said film clamp maintains said filmout of contact with said sealing element after the sealing operation hasbeen completed.
 6. The apparatus of claim 1, wherein said film clamp ismovable between a first pre-sealing position wherein said at least onesurface positions said film above and out of contact with said sealingelement and a second sealing position wherein said at least one surfacepositions said film in contact with said sealing element.
 7. Theapparatus of claim 6, wherein said film clamp is movable between saidfirst and second positions upon the application of pressure from saidtop jaw assembly.
 8. The apparatus of claim 6, further comprising atleast one spring in said bottom jaw assembly supporting said film clamp,and wherein said at least one spring is in a compressed state when saidfilm clamp is in said second position.
 9. The apparatus of claim 1,wherein said film clamp is substantially U-shaped and comprises a baseand a pair of opposite sides terminating in a respective pair of freeends.
 10. The apparatus of claim 9, wherein said opposite sides of saidfilm clamp taper inwardly towards said sealing element.
 11. A method forsealing film about an article, comprising: providing upper and lowersealing jaws movable with respect to each other, said lower sealing jawcomprising a sealing element and a film clamp; compressing said filmbetween said upper sealing jaw and said and lower sealing jaws so as tocause said film to contact said film clamp; and further compressing saidfilm between said upper sealing jaw and said lower sealing jaw so as tocause said film to contact said sealing element to seal said film. 12.The method of claim 11, wherein during said first compressing step saidfilm clamp is in a first pre-sealing position wherein said film clamppositions said film out of contact with said sealing element and duringsaid further compressing step said film clamp is in a second sealingposition wherein said film clamp positions said film in contact withsaid sealing element.
 13. The method of claim 12, further comprisingterminating compression of said film to cause said film clamp to returnto said first position.
 14. A method for sealing film about an article,comprising: providing a movable upper jaw and a stationary lower jaw,said lower jaw comprising a sealing element and a film clamp;positioning said sealing film between said upper and low jaw; movingsaid upper jaw towards said lower jaw and forcing said sealing film onsaid film clamp above said sealing element; and contacting said upperjaw and said lower jaw so as to force said film to contact said sealingelement to seal said film.
 15. The method of claim 14, wherein said filmclamp is movable between a first non-sealing position wherein said filmclamp positions said film out of contact with said sealing element and asecond sealing position wherein said film clamp positions said film incontact with said sealing element.
 16. The method of claim 15, furthercomprising terminating contact between said upper and lower jaws tocause said film clamp to return to said first position.